1. Field of the Invention
The present invention relates to a method of and an apparatus for growing a ribbon of crystalline silicon for use in solar cells or the like.
2. Description of the Related Art
Heretofore, it has been customary to lift a ribbon of single crystalline silicon in a crystal growth furnace with manual actions performed by the operator while the operator is visually observing the interior of the crystal growth furnace. Specifically, the operator first dips a seed crystal in a melt of silicon at a high temperature by moving the actuator of a crystal lifter in an opposite direction and adjusting the speed of the actuator and stopping the actuator while observing an enlarged image of the melt that is captured by a television camera and displayed on a display monitor. Here, melt is also called as xe2x80x9cfree meltxe2x80x9d, which is a state in which high temperature liquid phases of semiconductor material is freely movable in the crucible.
The operator manually stops a descent of the seed crystal by changing the speed of the actuator to a low speed when the seed crystal approaches the melt and then pressing the stop button when the operator judges that the seed crystal touches the melt. Since the movement of the seed crystal is manually controlled, the length of the tip end of the seed crystal which enters the melt has not been constant. The temperature of the melt is controlled by measuring the time in which the tip end of the seed crystal becomes melted off. However, because the time in which the tip end of the seed crystal becomes melted off depends on not only the temperature of the surface of the melt but also the distance that the tip end of the seed crystal enters the melt, the temperature of the melt cannot accurately be controlled. For this reason, the temperature of the melt when the crystal is lifted off the melt tends to become unstable, and the growth of quality crystals depends on the skill of the operator.
The operator brings the seed crystal into contact with the melt at a temperature which allows the seed crystal to be kept in contact with the melt for a certain period of time. Then, the operator presses a button to lower the temperature, cooling the melt to a temperature capable of growing a silicon crystal. The silicon crystal grows laterally into a wingout. For producing a ribbon of single crystalline silicon, the seed crystal is held in contact with the melt in a particular crystal orientation, and a silicon crystal starts to grow linearly from the area where the seed crystal contacts the melt in a direction perpendicular to the direction in which the seed crystal is to be pulled up, due to growth anisotropy of the crystal. When the wingout has grown to an appropriate length, it starts to be lifted from the melt, thus forming a ribbon of single crystalline silicon.
The surface of the melt has conventionally been given a bathtub-shaped temperature distribution in the direction of wingout growth. Unless the length of the wingout is controlled at the time of starting to pull up the wingout, the temperature of the melt may be too low or high, making it difficult to achieve good crystal growth. If the tip end of the seed crystal is not at the symmetrical center of the wingout when the wingout starts to be lifted, then the growing crystal is brought out of balance and rotates off the surface of the melt, terminating the crystal growth. In order to prevent the above difficulties from occurring, it is necessary for the operator to observe the length and asymmetry of the wingout with an accuracy of 1 mm through a viewing window on the crystal growth furnace, and press a button to start lifting the wingout at a suitable time. However, the operator needs to be highly skilled to judge the time to press the button because the viewing window is generally spaced from the surface of the melt by several hundreds mm.
For stably growing a long ribbon of silicon, it is desirable to grow the crystal at the center of a growth slot which is positioned in an upper central region of the crystal growth furnace and through which the ribbon of silicon passes. Heretofore, crystals are often grown at an angle to the vertical direction owing to slight accuracy deviations of the crystal lifter. Therefore, it has been customary for the operator to control the position of the crystal lifter to bring the crystal at the center of the growth slot while visually observing the crystal through the viewing window. During the crystal growth, consequently, the operator is required to operate the crystal lifter while observing the interior of the furnace at all times. Inasmuch as the accuracy with which to operate the crystal lifter depends on the skill of the operator, it is difficult to obtain quality crystals reliably at all times.
If the temperature of the melt, particularly the temperature of the surface of the melt, varies due to some disturbances occurring during the crystal growth, then the crystal which is growing is liable to suffer defects that tend to impair the crystal growth. When the crystal growth is impaired, it is necessary for the operator to take the crystal out of the furnace immediately and raise the temperature of the melt. When the operator finds the crystal growth impaired, however, it has been the conventional practice for the operator to press a button and remove the crystal from the furnace.
Inasmuch as it has heretofore been necessary for the operator to press a button according to a judgment based on a visual observation during the growth of a ribbon of crystal, the operator""s skill and labor has been required to produce such a ribbon of crystal. In addition, the operator""s visual observation tends to suffer an error in millimeters, resulting in a judgment error and a control action error. Furthermore, various operators have different skill levels, and the cost of production and productivity have been problematic with the conventional crystal growing process.
It is therefore an object of the present invention to provide a method of and an apparatus for growing a ribbon of single crystalline silicon at a low cost without depending on the skill of the operator.
According to the present invention, there is provided a method of growing a ribbon of crystal, comprising the steps of lowering a seed crystal toward a melt, detecting a contact between the seed crystal and the melt with an image captured by an imaging device, adjusting the temperature of the melt to keep a meniscus of the melt in contact with the seed crystal, lowering the temperature of the melt to create a wingout extending from the seed crystal, detecting the wingout with an image captured by the imaging device, and starting to lift a ribbon of crystal from the melt following the detection of the wingout.
Solid/liquid phase interfaces of the melt in contact with the seed crystal and the wingout are detected with an image captured by the imaging device, and various actions can be performed while observing the captured image. Therefore, time instances for making those actions, which have heretofore depended on the skill of the operator, can reliably be determined based on the image information. Thus, even the operator who is not sufficiently skilled or even an apparatus which is not attended and operates almost fully automatically, allows a ribbon of crystal to grow accurately. As a result, a quality ribbon of crystal can continuously be lifted from the melt at a low cost.
Preferably, the method may further comprise the steps of lowering the seed crystal by a predetermined distance, stopping the seed crystal for a given period of time measured by a timer, attempting to detect a contact between the seed crystal and the melt while the seed crystal is stopped, and repeating the lowering, stopping, and attempting steps until a contact between the seed crystal and the melt is detected. These additional steps ensure that the seed crystal will reliably be brought into contact with the melt. Particularly, when a time in which a meniscus of the melt contacted by the seed crystal is retained is detected, the temperature of the melt can be adjusted accurately to an optimum value, allowing the ribbon of crystal to be pulled up stably.
Preferably, the method may further comprise the step of detecting coordinates of the seed crystal in a region of contact between the seed crystal and the melt, and a length and symmetry of the wingout extending from the coordinates. This step makes it possible to easily and reliably recognize the time to start lifting the ribbon of crystal.
Furthermore, it is also preferable to detect the coordinates of the ribbon of crystal being lifted from the image captured by the imaging device, and adjust the relative position of a crystal lifter to lift the ribbon of crystal at a certain position. The operator is not required to attend the process to monitor the lifting of the ribbon of crystal, but the ribbon of crystal which is of stable quality can automatically be lifted.
According to the present invention, there is also provided an apparatus for growing a ribbon of crystal, comprising a container containing a melt of crystal, a crystal lifter for lifting a seed crystal away from the melt of crystal, an imaging device for capturing an image of a ribbon of crystal produced from the melt by the seed crystal, and a controller for controlling the shape of the ribbon of crystal, and the temperature of the melt based on the image captured by the imaging device.
Specifically, an apparatus for growing a ribbon of crystal according to the present invention has video cameras as an imaging device and an image processor which are combined with a crystal lifter, for producing desired measured data with a high accuracy in units of 0.1 mm, and optimizing a process of converting captured images into binary images and a process of recovering captured images from errors, thereby to detect the lifting of a ribbon of crystal highly accurately. Most of actions which have heretofore been performed by the skilled operator based on its experience can be automatized. The apparatus can be automatically controlled based on measured parameters obtained by image processing which are used as a reference for judgment, to grow a ribbon of crystal essentially in the absence of the operator""s intervention. As a result, it is possible to grow ribbons of crystal having a stable quality at an increased yield and a reduced cost.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.